In aviation, the take-off and landing phase of an aircraft is the most safety-critical part of a flight, because in particular large freight and passenger aircraft can take off or land only on surfaces (landing runway) correspondingly provided for the purpose, and owing to the vicinity of the aircraft to the ground in particular during the landing process, there is an increased risk of undesired ground contact and therefore an increased risk of an accident. During the take-off and landing phases, the movement directions of the aircraft relative to the ground (rolling on the take-off and landing runway, ground speed) or inertial speed vector must be aligned with the movement of the aircraft relative to the air (air speed or wind vector). In the event of crosswinds, this process leads to an increased risk of an accident.
In order that aircraft can land safely and smoothly even in the event of strong crosswinds, there are basically two different landing methods which pilots must be able to perform. These are firstly the de-crab method and secondly the sideslip method. In the de-crab method, the aircraft lands, aerodynamically in alignment with the wind, with a correction angle, that is to say the longitudinal axis of the aircraft (in general also the nose of the aircraft) is aligned with the wind direction, such that owing to the aircraft's airspeed and the correction angle, the drift resulting from the crosswind is compensated. Shortly before or shortly after touchdown, depending on the airline or operator, the pilot performs a maneuver to align the aircraft longitudinally with respect to the landing runway, in order that, after touchdown, the aircraft remains on the landing runway during the roll-out process or roll-out phase. Said maneuver requires a great deal of experience on the part of the pilots and can destabilize the roll-out process of the aircraft on the landing runway, wherein if the de-crab method is implemented incorrectly, considerable forces, in particular transverse forces, are exerted on the nose undercarriage or main undercarriage, as a result of which, to ensure the required level of safety, correspondingly robust dimensioning of the undercarriages is necessary, which results in a large undercarriage weight.
The sideslip method is a landing method for crosswind situations in which the pilot steers the aircraft in the direction of the wind by means of the aileron, such that the wing facing the wind is “left hanging”. In order that the aircraft is not turned into the wind by the aileron deflection, the pilot counter-steers with the rudder and keeps the aircraft on course along the landing runway, such that, as a result, said state is also referred to as “crossed rudder”. As a result of the fact that the wings which are inclined relative to the wind are left hanging, the drift owing to the crosswind is compensated, such that landing without a correction angle is possible. A disadvantage of said method is in particular the fact that, in the event of very strong crosswinds and therefore a very steeply hanging wing, there is the risk during touchdown of the aircraft on the landing runway that the hanging wing (or the engine mounted under the wing) makes contact with the ground, which can lead to severe accidents.
During a landing approach, hanging angles of greater than 5° must be avoided in order to prevent contact of the wing tips with the landing runway.
To compensate for the disadvantages of the de-crab method, specifically the increased transverse forces during touchdown, U.S. Pat. No. 6,722,610 B1 describes a special turnable aircraft undercarriage in which each individual wheel of the undercarriage or of the complete undercarriage system of the aircraft can be pivoted separately about the longitudinal axis (turning axis is the yaw axis) in order thereby to compensate the correction angle during landing in accordance with the de-crab method (so-called crab angle) by turning the wheels parallel to the movement direction of the aircraft over ground. For this purpose, on the basis of the actual movement direction of the aircraft over ground, the direction of the longitudinal axis, which in the de-crab method deviates from the movement direction, is determined, wherein the crab angle is determined from the difference between the actual movement direction of the aircraft over ground and the direction of the longitudinal axis. Each of the wheels of the aircraft is now turned through precisely this crab angle relative to the longitudinal axis, such that the running direction of the wheels is aligned parallel to or in the direction of the movement of the aircraft over ground.
The undercarriage is thus acted on by no forces other than those that would arise during a normal landing without the de-crab method, because the wheels are aligned in the movement direction of the aircraft. A disadvantage here is however in particular the fact that the touchdown point of the aircraft on the landing runway may shift as a result of spontaneous changes in wind direction or wind speed, such that the aircraft does not touch down exactly centrally on the landing runway, which ultimately increases the risk of an accident, which may in the worst case result in the aircraft leaving the landing runway.